The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
The automotive industry typically applies an aluminized coating to the surface of hot stamped boron steel for the purpose of improved oxidation resistance. More specifically, the aluminized coating reduces oxidation of the boron steel during austenitization at temperatures of 880 to 930° C. Unfortunately, the aluminized coating limits many hot stamping process parameters including, but not limited to furnace dew point, furnace residence time, furnace temperature, and furnace dwell time.
As the aluminized boron steel is austenitized, the aluminized coating thickness increases as the Aluminum diffuses into the steel. Similarly, the Iron diffuses into the aluminized coating. The coating layer at the interface between the steel and aluminized coating represents the inter-diffusion layer (IDL). The resultant thicknesses for the aluminized coating and the IDL depend on process parameters including but not limited to the furnace dew point, furnace temperatures, and furnace dwell or residence times. The resultant thicknesses correlate to the weldability and paintability of the hot stamped part.
Current industrial Quality Control (QC) practice has been to assess and inspect development and growth of the aluminized coating, following the hot stamping process, by measuring the IDL thickness at the steel-coating interface by using optical microscopy (i.e. optical light microscopy or OLM). OLM is also used to measure the total thickness of the aluminized coating. These QC methods include sampling the hot stamped part followed at least by the sample preparation steps of:                Mounting the samples with an adhesive to create metallographic samples;        Grinding the metallographic samples;        Polishing the metallographic samples; and        Etching the metallographic samples;        
Following sample preparation, optical microscopic analysis is performed on the etched metallographic samples so that measurements of the thickness of interest are determined. Some of the limitations of these QC methods include:                Long lab processing times, about four (about 4) hours from sampling to OLM analysis, to obtain the coating thickness measurement results;        Significant material is wasted or scrapped to obtain acceptable surface quality for OLM measurements;        The thickness measurements are highly dependent on the verticality of the adhesive mounting as slight inclinations from vertical result in large increases in thickness measurements (parts out of tolerance) potentially resulting in unnecessary scrapping or rework;        The IDL thickness measurements are dependent on the quality of chemical etch, the calibration accuracy of the optical microscope, operator experience, the location on the sample, and various other factors;        The total coating thickness measurements are dependent on the quality of the metallographic sample preparation, particularly near sample edges where minor preparation mistakes lead to large inaccuracies in the measurements; and        The QC OLM measurements are taken very locally (approximately 0.3 mm) at high magnification (500×) and often do not represent the entire surface of the hot stamped part.        
The present disclosure addresses the issue of time-consuming QC procedures in measuring the various thicknesses and constituents of aluminized coatings on the surface of hot stamped boron steels, among other issues related to measuring and characterizing coatings on various substrate materials.